1. Field of the Invention
The present invention generally relates to an FM modulator. More specifically, the invention relates to a system for reducing undesirable sounds associated with the FM modulator turn on.
2. Description of Related Art
One application of an FM modulator includes modulating stereophonic composite audio signals from peripheral devices such as, DVD players, allowing the resulting RF signal to be played by a standard stereo radio receiver. The ability to use peripheral devices with a standard radio receiver can provide a significant cost savings for automotive applications. However, it is recognized that a certain set of chirps and pops are created in association with FM modulator turn on. These sounds are annoying and distracting to most consumers.
One undesired sound is an annoying whistle from high to low-frequency. FM modulators employ a phase lock loop (PLL) which facilitates selection and precise control of frequency of the FM modulator output signal. While the operating frequency is precisely controlled, there is a time interval just after FM modulator turn on that the RF output frequency sweeps over some wide range. The whistle is due to the heterodyning of the sweeping RF output signal of the FM modulator with the local oscillator of the vehicle radio receiver. Therefore, the whistle is present shortly after FM modulator turn on, and stops once the FM modulator PLL is locked onto the operating frequency of the FM modulator.
Additionally, popping sounds are created when the RF level input to a radio receiver changes abruptly, such that the change in the RF input level occurs faster than the response time of the vehicle radio receiver AGC system. For example, if the RF level is switched on very quickly, the AGC of the receiver will not be able to track the input signal, therefore the end result will be a large amplitude popping sound.
Sudden sounds including pops, whistles and abrupt volume changes are especially noticeable to the consumer when associated with a particular event such as turning on a radio. Consumers perceive pops and whistles as being indicative of a poor quality radio, thereby making the sounds highly undesirable for manufacturers.
In view of the above, it is apparent that there exists a need to eliminate undesired sounds during FM modulator turn on. Satisfying that need is one of the primary objects of the present invention.